It has been commonly assumed that the leading cause of disease transmission among animal herds is by contact of infected animals with non infected animals. But since this theory has been so accepted there is little attention payed to other possible causes of disease transmission such as airborne spreading. This article describes instances where diseases have been transferred not by the introduction of infected animals but by aerial transmission. Foot and mouth disease in the UK was found to be caused by the introduction of infected animals in 5% of cases and 79% because of local spreading. In 1997 the epidemic of hog cholera was only found to be spread because of introduction of infected animals in 3% of the cases. Local spreading was the cause of the disease spreading in 36% of the cases. Porcine reproductive and respiratory syndrome is another common disease among swine herds. In an outbreak in Quebec, 44 cases of PRRS were detected, but the multiplier herd  had never tested positive for PRRS meaning that none of these cases were because of the introduction of an infected animal. Enzootic pneumonia is another common disease worldwide in the pig industry. A outbreak in Quebec was from a completely negative herd source leading to the belief that the herds were infected by area spread, or neighborhood infection. These events suggest that aerosol transmission should be considered as a likely source of disease spreading along with infected semen and introduction of infected animals.